Over the recent years, the functionality and complexity of wireless communication devices such as mobile handsets or computer dongles has been increasing along different axes. Wireless communication devices may have multiple built-in wireless systems such as a cellular modem, Bluetooth® short range wireless technology, GPS/GNSS, and WiFi/802.11. Handsets and dongles are usually referred to in the 3GPP specifications as User Equipment (UE).
A cellular modem in a UE will typically be multimode (supporting all or a subset of GSM/GPRS/EDGE/WCDMA/TD-SCDMA/TD-LTE/FD-LTE), multiband, and may additionally support receiver (Rx) and/or transmitter (Tx) diversity. Providing such a complex set of wireless functionality within a handset is a challenging engineering and integration effort in order to meet the significant constraints in terms of size, cost, and power consumption when compared to those of existing simpler UEs supporting a much smaller set of functionality.
A UE can be requested to operate in Full Duplex (FD) mode, where Rx and Tx operations are concurrent, or in Half Duplex (HD) mode, where Rx and Tx operations never occur simultaneously. In a Full Duplex Frequency Division Duplex (FD-FDD) system, the UE receiver and transmitter operate simultaneously on different frequencies, as shown in FIG. 1. The different frequencies provide the necessary separation between uplink and downlink signal paths.
One method of providing Half-Duplex operation while using the same carrier frequency is Time Division Duplex (TDD) where the time domain provides the uplink and downlink separation. Alternatively different carrier frequencies can be used for a Half Duplex FDD (HD-FDD) where uplink and downlink communications are not only on distinct frequencies but are also separated in the time domain. By scheduling UEs at mutually exclusive times transmission resources can be fully utilized. The base station is effectively working in full-duplex while the UEs operate in HD-FDD mode.
FD-FDD operation usually requires a duplexer on the Tx antenna as well as one filter on the diversity antenna for each band. The more FD-FDD bands one UE supports, the more filters and duplexers it requires, which is only sustainable if the number of bands supported remains small. Duplexers and filters are commonly designed using Surface Acoustic Wave (SAW) technology or Bulk Acoustic Wave (BAW) technologies. Furthermore, additional package and Printed Circuit Boards (PCB) costs are required because of bulky components. In short, it is foreseeable that with developments on a larger number of multiple networks, more costs and complexities would occur in the designs of UE if FD operation is a requirement. Both WCDMA/HSPA+ and FDD LTE operate in full duplex mode (some standardization work has already been performed in 3GPP to specify the HD operation mode for FDD LTE but this work is incomplete). For more information, see 3GPP contribution R1-113438, “Identification of standards impacts of low cost LTE UEs”, IP Wireless Inc., 3GPP TSG RANI 67 bis.
On the contrary, HD-FDD and HD-TDD solutions are amenable to filter-less receiver implementations, which can be implemented using a simpler, more cost-efficient approach whose complexity does not scale with the number of HD frequency bands supported. For example, see “Highly Integrated and Tunable RF Front Ends for Reconfigurable Multiband Transceivers: A Tutorial”, by H. Darabi et al., IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 58, NO. 9, SEPTEMBER 2011. GSM/GPRS/EDGE UEs typically operate in HD-FDD mode, whereas TD-LTE and TD-SCDMA/HSPA operate in (TD-) TDD mode.
Study has been done on different aspects of the transceiver and front end module complexity associated to multiband, multimode, multi-antenna UEs. Such study can be found in “Half duplex/full duplex LTE 800 handsets”, by GSM Association Spectrum Management Group on 2008 focusing on the RF cost and performance implications of supporting half duplex FDD for LTE 800 in Europe@, RTT UK, and “3G-4G Evolution Multimode Modems”, by Qualcomm 4G World on 2011, “LTE Spectrum Strategies and Forecasts to 2016” by Informa Telecoms and Media, Webinar, on July 2011, and ‘WCDMA for UMTS’, by Harri Holma and Antti Toskala, 5th edition.
FD-FDD Operation in Legacy Transceiver Implementations
A UE typically consists of a cellular baseband modern, a power management unit, a radio transceiver chip, one or more radio amplifiers, and a front end module. The front end module contains a number of antennas, a set of passive filters and duplexers, and a set of antenna switches.
On a typical legacy receiver implementation for a front end module supporting multi-mode, multi-band, multi-antenna in one device, complexity grows linearly with the number of supported full duplex and half duplex frequency bands (NFD and NHD, respectively) and supported number of antennas (NA) as follows:                NA antennas        NPA power amplifiers (PA), typically fewer than (NFD+NHD). Generally one power amplifier is able to support a group of frequency bands located over the same frequency region. Wideband PAs are now becoming available, so this number can be small (for example, two wideband PAs and two PA switches might be needed on a handset covering between 700 and 2600 MHz).        NFD duplexers and (NA−1)×NFD Rx filters to enable concurrent Rx/Tx for all the Full Duplex operation paired FDD bands (FD-FDD), on the assumption that only one antenna is used for Tx operation and all NA antennas are used for Rx operation.        (NA×NHD) Rx filters to cover all the bands in which HD operation is performed.        NA SPMT (Single Pole Multiple Throw) antenna switches, each with between (NHD+NFD+NPA) throws. A larger number of throws increases cost and degrades performance.        The PCB footprint for the modem grows with the number of components.        The total number of Rx paths (NA×(NHD+NFD)) requires an equal number of Low-Noise Amplifiers (LNAs) on-chip and also increases the transceiver chip pin count. These factors also increase the total cost.        
Advantages and limitations of filter-less transceiver implementations
Recent developments to radio transceiver design have enabled the implementation of filter-less front end modules, which are characterized by the absence of any Rx filter between each antenna and the associated Rx part of the transceiver. This does not come without a challenge (as detailed in “Identification of standards impacts of low cost LTE UEs” cited above) since such receiver implementations will require far more linear LNAs in order to avoid receiver de-sense by out-of-band Rx blockers, which are no longer being attenuated by a Rx filter. Filter-less implementations for GSM, GPRS and EDGE are already in existence.
As shown in FIG. 2, a dual-PA traditional implementation is supporting the 4 bands in the 700-1000 MHz region, 4 bands in the 1700-2.1 GHz region and 4 bands in the 2.2-2.6 GHz regions might require as many as 2×12=24 Rx filters, one SP14T switch, one SP12T switches and 24 LNAs on the transceiver. The cost of supporting such a large number of non-integrated components is so significant that the concept of a global phone supporting all bands used in different world regions and benefitting of economies of scale is fairly unlikely to become a reality.
On a 100% filter-less implementation the (NA×NHD) filters/duplexers and equal number of associated LNAs would be replaced by a much smaller number wideband LNAs. The number would be proportional to band regions and number of Rx antenna s being supported. For example, as shown in FIG. 3, a HD transceiver with 2 Rx antennas covering 4 bands in the 700-1000 MHz region, 4 bands in the 1700-2.1 GHz region and 4 bands in the 2.2-2.6 GHz regions would typically require no Rx filters, two PAs, no PA switches, one SP5T antenna switch, one SP3T switch and only 3 pairs of wideband LNAs.
Unfortunately filter-less implementations are only applicable to HD-FDD and HD-TDD operation but not to FD-FDD. As a result of this, WCDMA/HSPA+ and FDD LTE are not in a position to benefit from filter-less implementation. This is because it is not possible for the LNA to tolerate the presence of the local Tx signal which acts as a really strong out-of-band blocker whose average power is in the order of 24 dBm. Unless some filtering is provided to bring this power down to lower levels that the LNA will tolerate, it is impossible to operate in full duplex FDD mode.
Partial Filter-Less Implementations—Feasibility
While a part-filter-less solution applied to the HD subset of bands (i.e. those dedicated to GSM/GPRS/EDGE, TD-SCDMA, and TD-LTE) does provide some benefit, such benefit is still nowhere near the cost/performance/power efficiency attained by a fully filter-less transceiver described in the previous section. Continuing with the previous example, if 50% of the supported bands operate in HD mode (2 out of 4 on each band region), then the settings are the following:                HD bands: zero duplexers or Rx filters, two SP3T switches and 3 pairs of LNAs        FD bands: 8 duplexers and 8 Rx filters, and associated 16 LNA pairs        One SP11T and one SP10T antenna switches        Two PAs and small PA switches        
Therefore a hybrid solution as shown in FIG. 4 will still require a total of 2 antennas, 6 duplexers, 6 Rx filters and 18 LNAs, which is very unlikely to happen for cost-sensitive high-volume consumer markets.
The preceding examples illustrate how FD operation is an obstacle for the development of economies of scale when addressing a large global cost-sensitive market with minimal UE design customization. This is due to the cost of addressing regional variations through Rx filter/duplexer customization for the relevant regional subset of FD-FDD bands.
Therefore, there is a desire to further expand and support the half duplex operation for benefiting from the advantages discussed above.